El Hierro, nicknamed Isla del Meridiano ("The Meridian Island"), is a Spanish island. It is the smallest and furthest south and west of the Canary Islands, in the Atlantic Ocean off the coast of Africa.

The name of the island is derived from the Guanche language toponym Hero, which by the process of folk-etymology was transformed into "Hierro," meaning "iron" in Spanish, due to its similarity to that word. Thus, Ferro (Latin ferrum, "iron") was and is used as an alternative name for the island. It is the name for the island in other languages, including French, German, and Danish. Pliny the Elder, who used Juba II as his source, names a series of Canary Islands, and it is believed that his Capraria may have been Hierro.

The ancient natives of the island, called bimbaches, were conquered by Jean de Béthencourt –more through the process of negotiation than by military action. Béthencourt had as his ally and negotiator Augeron, brother of the island's native monarch. Augeron had been captured years previously by the Europeans and now served as mediator between the Europeans and the Guanches. In return for control over the island, Béthencourt promised to respect the liberty of the natives, but he eventually broke his promise, selling many of the bimbaches into slavery. Many Frenchmen and Galicians subsequently settled on the island. There was a revolt of the natives against the harsh treatment of the governor Lázaro Vizcaíno, but it was suppressed.

The island was known in European history as the prime meridian in common use outside of the future British Empire. Already in the 2nd century A.D., Ptolemy considered a definition of the zero meridian based on the western-most position of the world, giving maps with only positive (eastern) longitudes. In the year 1634, France ruled by Louis XIII and Richelieu decided that Ferro's meridian should be used as the reference on maps, since this island is the most western position of the Old World and also thought to be exactly 20 degrees west of Paris, so indeed the exact position of Ferro was never considered. Old maps (outside of Anglo-America) often have a common grid with Paris degrees at the top and Ferro degrees offset by 20 at the bottom. Louis Feuillée also worked on this problem in 1724.

It was later found that the actual island of El Hierro itself is in fact 20° 23' 9" west of Paris, but the Ferro meridian was still defined as 20 degrees west of Paris.

The Ferro meridian is 17° 39' 46" west of the Greenwich meridian.